If you've just built yourself a brand-new PC or simply bought a new kit for your current PC, the first thing you'll want to do is enable the XMP or EXPO profile in the BIOS. While these memory profiles are designed to run on a wide variety of system conditions, there are certain cases where your RAM will simply refuse to boot with its rated speed.
Stability with XMP and EXPO usually comes down to your motherboard, CPU, and specific DIMM revisions (different ICs or slightly distinct PCB layouts on the same kit). Silicon variance (especially on the CPU's memory controller), differences in BIOS training logic, and DIMM training logic could mean that the primary memory profile is too aggressive for your PC.
Your BIOS isn't up to the task
Early BIOS versions especially struggle with compatibility
When you enable XMP or EXPO on your RAM kit, the motherboard has to read the XMP/EXPO profile data stored on the DIMM. It then has to apply the listed timings, voltage, and frequency, and run "memory training." This process depends on the BIOS' memory-training firmware (AMD AGESA or Intel MRC) to correctly interpret the XMP/EXPO fields. However, new or outdated BIOS, or those with a vendor-side bug, may not implement new memory standards correctly. This can also happen with new kits and when RAM manufacturers switch out the ICs in their kits due to supply chain rotation and availability. Different ICs (Hynix, Micron, Samsung, etc.) possess distinct voltage/timing tolerance characteristics, and the BIOS memory training firmware needs to be updated for that specific IC revision.
In essence, if the BIOS firmware is not updated to handle newer memory modules, memory training can fail, even if the memory kit itself is good. BIOS training failures are more likely to affect high-density DIMMS or 2 DPC (2 DIMMS per channel) configurations, so you should especially keep your BIOS updated in such cases. These updates are provided by AMD/Intel and applied by the motherboard vendor, and in BIOS update logs, you'll see mention of AGESA/MRC.
You didn't win the silicon lottery
No silicon is equal to the other
Integrated memory controllers inside the CPU, which control memory communication, speeds, and timings, can differ between CPUs, even in the same SKUs. This is because silicon, which the IMC is made of, naturally shows variance in manufacturing. Any tuning you apply to your RAM kit must be supported by the controller, but this means that those advertised XMP/EXPO speeds you see may run on some PCs but not on others. Signs of an IMC limitation include POST (power on self-test) failure, boot loops, BSODs, or application crashes under memory-heavy workloads—when you use the fastest profile.
You're using high-density or quad-DIMM kits
Greater load, less stability
Higher density DIMMs (8 GB vs 16 GB) and the use of four DIMMs instead of two lead to greater electrical load on the channel. A greater load reduces timing margin and degrades signal integrity compared to lower-density modules or 1 DPC configurations. Now, you may be asking: Don't those DIMMs already account for the increased load and reduce XMP/EXPO speeds? The answer is: yes. They do. But they also lower the margin of error when it comes to stability. This means that, generally, there's a lower guarantee of successfully running such kits at their advertised speed compared to kits with reduced electrical load.
The memory subtimings are too tight
XMP/EXPO profiles don't control these
XMP and EXPO profiles only define a handful of primary timings (tCL, tRCD, tRP, tRAS), while all the secondary and tertiary timings are left for the motherboard’s memory-training algorithm to generate automatically. This is another reason that the exact same RAM kit and CPU can behave differently across boards. ASUS, MSI, Gigabyte, and ASRock all use their own auto-rules, and these rules change between BIOS versions.
If these auto-calculated subtimings end up being too aggressive for the memory controller (for example, tRFC or tFAW being tighter than the memory ICs can tolerate), the kit may fail training and silently fall back to a lower memory speed, even though the RAM itself is rated for higher speeds. This is why you'll see BIOS update logs mention “improved memory training stability." It means that the vendor has adjusted the automatic subtiming behavior for better compatibility.
Those XMP/EXPO speeds aren't always guaranteed
Most users can run their memory kits at the speeds mentioned on the box, but some can't. This comes down to various factors, including the IMC, BIOS training, DIMM configurations, and voltage settings. If you're one of those unlucky people, troubleshooting involves selecting a different profile, lowering the speed or primary timings, and ensuring that the BIOS is updated. Also, a good rule of thumb is to only buy memory kits that are supported by your motherboard under the Qualified Vendors List (QVL); though note that it does not completely guarantee compatibility either.